Actually, given the taint stack it's more likely this is caused by Xloot_Master's use of StaticPopupDialogs, which taints at least one of the 4 entries each time it confirms a loot assignment. Then later when the talent UI uses that same entry to reset talents it dies.
Xloot_Master does use preferredIndex = 3, but that just makes the problem less likely, not impossible.

As I replied to your other ticket, this is not a XLoot issue. This is a Blizzard issue. Blizzard's InterfaceOptionsFrame taints whenever a addon adds itself to the Interface options/Addon menu. It doesn't matter what addon - XLoot just gets blamed because it touched it last. Try disabling it, you'll get the error on another addon so long as they add a option panel.